This invention relates to a method and apparatus for the filtration of a liquid.
Methods and apparatus for the filtration of liquids have many and varied applications, including without limitation the filtration of suspended and/or colloidal solids from waste water. Apparatus and methods have been developed which filter liquids by flow through a bed of particulate filter media (e.g., sand) in which the particulate filter media is continuously washed and recycled in the filter. See Hjelmner et al., U.S. Pat. Nos. 4,126,546 and 4,197,201.
In the filter apparatus shown in U.S. Pat. No. 4,060,484, the filter bed of sand has an inclined upper surface which has been created by the use of a baffle plate. The liquid to be filtered flows through the filter bed by entering the bed of sand through this inclined upper surface, passing through the filter bed, and exiting through a screen located near the exterior of the filter vessel. Sand from the filter bed is withdrawn from the bottom of the bed and raised above the top of the bed using air lift, causing the filter bed media to move in a downward motion toward the area from which the sand is being withdrawn. A portion of the liquid introduced to the filter is used as wash water to wash the sand which has been withdrawn from the bottom of the filter bed. The wash water, containing the contaminates removed from the washed sand, flows out of the filter apparatus.
It was recognized in U.S. Pat. No. 4,060,484 that particles of sand flowing downward over an inclined surface have a tendency to "classify" themselves according to size. It was stated in U.S. Pat. No. 4,060,484 that "Smaller particles tend to remain at the top of the slope while larger particles roll down the slope." This gives a structured effect to the filter bed shown in U.S. Pat. No. 4,060,484, in which coarser particles of sand would migrate toward the inlet side of the filter bed, and finer particles of sand would migrate inwardly therefrom. The water outlet side of the filter bed had particles of full size range because sand entering the filter vessel adjacent the exterior of the filter vessel would flow vertically downward without segregation.
In Hering, U.S. Pat. No. 4,891,142, an attempt was made to increase the efficiency and effectiveness of the filter shown in U.S. Pat. No. 4,060,484. In the filter shown in U.S. Pat. No. 4,891,142, as in the filter shown in U.S. Pat. No. 4,060,484, the filter bed media is withdrawn from the base of the filter bed, washed, and resupplied to the top of the filter bed. However, in the filter shown in U.S. Pat. No. 4,891,142, the filter media forms a peak shape which descends downwardly and radially outward from the center of the filter. The liquid to be filtered passes through the peak shape and is filtered downwardly through the filter bed. The peak shape forms an inclined surface which also "classifies" the filter bed media by size, the coarser particles of filter bed media tending to roll down the slope of the peak while the finer particles tend to flow vertically downward through the peak and the filter bed. The liquid to be filtered flows first through the outer layer of coarser filter media and then through the layer of finer filter media, exiting through a screened filtrate enclosure which is located toward the bottom of the filter bed in the center.
In Hering, U.S. Pat. No. 5,173,194, it was stated that downflow filters having a screen or other filter media retaining device, such as are shown in U.S. Pat. Nos. 4,060,484 and 4,891,142, are subject to clogging and blockage, requiring periodic removal of the filter bed media to permit cleaning or replacement of the screen or other filter media retaining device. The filter shown in U.S. Pat. No. 5,173,194 includes a centrally located liquid collection chamber having a liquid-impervious top or hood attached to liquid-impervious sidewalls with an open bottom. While this overall design is that of a downflow filter, the open bottom and liquid-impervious top and sidewalls of the liquid collection chamber obviate the need for a screen or other porous filter media retention device.
One aspect of this invention is to provide a self regenerating particulate filter in which the contaminated filter media is cleaned by a countercurrent flow of filtrate from the filter.
Another aspect of this invention is to provide a moving bed filter which eliminates the "dead zone" of filter media in the filter bed and has a substantially reduced volume of filter media located below the gas lift conduit.
Yet another aspect of this is invention to provide a moving bed filter in which the regeneration zone for the filter media is separated from both the inlet to and the outlet from the filter by the filter bed.
Other aspects and advantages of the invention will become apparent from the drawings and description which follow.